Original Russian Text N. N. Nevedrova, E. V. Pospeeva, A. M. Sanchaa, 2011, published in Fizika Zemli, 2011, No. 1, pp. 63-75


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ISSN 1069 3513, Izvestiya, Physics of the Solid Earth, 2011, Vol. 47, No. 1, pp. 59–71. © Pleiades Publishing, Ltd., 2011.

Original Russian Text © N.N. Nevedrova, E.V. Pospeeva, A.M. Sanchaa, 2011, published in Fizika Zemli, 2011, No. 1, pp. 63–75.

59

INTRODUCTION



The Institute of Petroleum Geology and Geophys

ics, Siberian Branch, Russian Academy of Sciences, is

carrying out complex geological and geophysical sur

veys in the Mountain Altai. These studies were consid

erably expanded after the destructive Chuya earth

quake with a magnitude of 7.5 on the Richter scale

that occurred on September 27, 2003. This was the

strongest event over the instrumental period of seismo

logical observations. The focal zone of the earthquake

overlaps the territory of the Chuya and Kurai Depres

sions, and the North Chuya Range. The main earth

quake rupture is well observed in the western part of

the Chuya Depression as a discontinuous belt of local

fractures, landslides, and ground displacements. It was

decided to carry out a complex electromagnetic survey

over a test area in the western closure of the Chuya

Depression, where the array electromagnetic mea

surements with controlled and natural sources were

deployed (Fig. 1).

Since no magnetotelluric sounding (MTS) had

been carried out in the Mountain Altai until recently,

the main goal of our study was to reconstruct the deep

geoelectric cross section of the lithosphere according

to the MTS data, and to refine the structure of the sed

imentary cover and the upper portion of the paleozoic

basement using a complex of MTS and near field

transient electromagnetic sounding (NF TEMS)

methods.


The present work also addresses another challeng

ing issue, which is urgent for all seismically active

regions including the Mountain Altai. It is studying

the time dynamics of the geoelectrical parameters of a

rock massif that underwent strong seismic impact

[Nevedrova, 2007]. There is a considerable amount of

archival electric data to support modern studies. These

data include vertical electric sounding (VES) and NF

TEMS results obtained for the Altai depressions in the

latter half of the 20th century. These data were used to

determine the geoelectrical parameters of the environ

ment before the destructive earthquake of 2003.

The efficiency of the electromagnetic monitoring

of geodynamical processes undoubtedly depends on

the detailed studies of the geoelectric structure to

which the present paper is primarily devoted.

FIELD TECHNIQUE

The controlled source electromagnetic sounding

(NF TEMS and VES) were carried out in the western

part of the Chuya Depression in a set of profiles

(Fig. 1). Figure 1 depicts the profiles and observation

sites of all electromagnetic measurements. NF TEMS

measurements were implemented as the sounding

with induction excitation of the field and recording the

time derivative of the vertical component of the mag

netic field (

Hz/∂t) in the coaxial loop configuration.

We note that in case of induction excitation and

recording, the high resistivity screens have no effect,

and the influence of local near surface heterogeneities

is weak. These factors are also important in the field

measurements on the territory of intermontane tec

tonic troughs, where the upper part of the cross section

contains insular permafrost and lenses of coarse

grained deposits. The side length of the transmitter

loop was 400 m, and the same was the spacing of mea

surement sites. The average distance between NF

TEMS profiles was 2–4 km.

Based on the geophysical interpretation, it has

been established earlier that the controlled source

electromagnetic methods in the geological conditions

of the Mountain Altai provide an exploration depth of



Interpretation of Complex Electromagnetic Data in Seismically 

Active Regions: Case Study of the Chuya Depression, Mountain Altai

N. N. Nevedrova, E. V. Pospeeva, and A. M. Sanchaa

Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch, Russian Academy of Sciences, 

pr. Akad. Koptyuga 3, Novosibirsk, 630090 Russia

Received January 12, 2010



Abstract—A procedure for the simultaneous interpretation of magnetotelluric and near field transient elec

tromagnetic sounding (MTS and NF TEMS, respectively) data is proposed. The advantages of the complex

interpretation are demonstrated by specific examples. In accordance with the interpretation of the field data,

geoelectrical sections of the lithosphere in the western part of the Chuya Depression are constructed. A

reduction in the depth to the conductive crustal layer in the epicentral zone is found, and the geoelectrical

boundary in the upper part of the paleozoic basement is revealed.



DOI: 10.1134/S1069351311010083

60

IZVESTIYA, PHYSICS OF THE SOLID EARTH

  Vol. 47 

  No. 1   2011

 NEVEDROVA et al.

up to 1–2 km. In order to increase the depth of inves

tigation and the informativeness of the geoelectrical

studies in the epicentral zone of a large earthquake, an

average scale MTS survey was carried out over the

range of periods from 0.003 to 6000 s. These MTS

measurements were conducted using the new genera

tion MTU–System–2000 (Phoenix Geophysics,

Canada) equipment provided with the software for raw

data processing (Satellite Synchronized MT, the

SSMT).

An extended MTS profile was acquired in the west



ern part of the Chuya Depression. The profile starts in

the southwest mountain framing of the depression, the

South Chuya Range, and ends in its northern part, in

the region of the Chagan–Uzun massif. The measure

ments were carried out at 23 stations with an average

spacing of 2 km. In the zone of tectonic deformations

of the earthquake the distance between the MT sites

was reduced to 1 km; the MTS stations in this region

were coincident with the NF TEMS sites.

Rectangular receiving setups consisting of

grounded receiving lines 

Е

х

,  Е



у

, and three magnetic

sensors  Н

х

,  Н



у

, and Н



z

 were used for registration of

magnetotelluric variations. The length of the receiving

electric lines was 100 m. This is the most suitable

length providing optimal signal to noise ratio in the

survey region. The time of recording was 19–22 h.

PROCESSING AND INTERPRETATION

OF THE COMPLEX EM DATA



Near Field TEM Data

At the first stage of processing, the field NF TEM

data acquired in the observation profiles was consid

ered. The entire volume of measurements was ana

lyzed; each apparent resistivity curve was analyzed

individually. The quality of measurements and possi

ble data corruption were assessed; the character of the

changes in the resistivity curves along the profile and

their correlation with each other were analyzed; and

pr.3 (NF TEMS)

pr.4 (NF TEM)

pr.5 (NF TEMS)

pr.6 (NF TEMS)

pr.7 (NF TEMS)

pr.8 (NF TEMS)

bel 18


bel 19

bel 20


bel 21

bel 22


bel 23

bel 14


bel 13

bel 12


bel 11

bel 15


bel 16

bel 17


bel

2

reg 21



reg 22

reg 20


reg 19

reg 18


reg 17

reg 16


reg 15

reg 11


reg 12

reg 13


reg 14

mtz 1


20

40 km


0

20

(1)



(2)

(1)


(2)

1

2

3

4

N

Fig. 1. The map of the actual surveys. Profiles and sites of electomagnetic observations in the western part of the Chuya Depres

sion: 1 NF TEMS sites, archival and present day; 2 VES sites; 3 MTS 2007

−2008 sites, present day and acquired by the Krasno

yarsk Research Institute of Geology and Mineral Resources; 4 MTS sites, 2009.


IZVESTIYA, PHYSICS OF THE SOLID EARTH

  Vol. 47 

  No. 1   2011

INTERPRETATION OF COMPLEX ELECTROMAGNETIC DATA

61

the main regularities in the transient process at differ



ent segments of the profile were revealed. Then, the

entire volume of the NF TEM field data was processed

using the interactive computer systems for interpreta

tion and computer modeling of nonstationary electro

magnetic fields. Two automated systems—ERA and

EMS program complexes, designed in the Laboratory

of Electromagnetic Fields, Institute of Petroleum

Geology and Geophysics, Siberian Branch, Russian

Academy of Sciences [Epov, 1990; Khabinov, 2009],

were applied. The ERA program complex is a universal

interactive system to work with the data of transient

electromagnetic sounding. It should be noted that the

EMS interpretation system is the development and

extension to the ERA program complex for modern

computers; it has good potential for using new NF

TEMS modifications and new visualization tech

niques. Both systems allow processing and interpreta

tion of the field data of active source electromagnetic

sounding using the models of laterally homogeneous

media.


Construction of the basic interpretation model is

the most important step in the computer processing of

the NF TEMS measurements. We invoked additional

a priori information for this purpose. The available

data for the existing wells were analyzed and general

ized.


Sedimentary filling of the depression took place at

the same geological time over the entire territory;

therefore, although the majority of the wells are

located in the central part of the Chuya Depression,

the same drilling data can be used also for the interpre

tation of measurements in the western part of the

depression. The average drilling depth is relatively

small (200–300 m). Only a few wells penetrated the

Paleozoic basement [Nevedrova, 2001]. Examining

the cross sections of these wells, we can trace the

changes in the lithological composition of the medium

with depth. The upper part of the cross section is com

posed of the coarsest gravel and pebble deposits that

become finer grained with depth. In the lowermost

part of the section, the basement rocks are usually

overlain by thin bedded clays and close grained sand

stones without coarse grained material. The same well

data can be used to estimate the thickness of all litho

logical complexes and the total thickness of the sedi

mentary filling, which makes it possible to unambigu

ously determine the electric resistivity of the identified

layers and, thus, to solve the questions concerning the

equivalence of the geoelectrical models.

It was ascertained that the lowest resistivity values

are typical for thin layered formations: recent and

Paleogene–Neogene clays, aleurolites, and argillites.

The electrical resistivity (ER) of these sediments varies

from 5 to 50 

Ω m. Among the Paleogene–Neogene

rocks, sandstones of the Tueryk suite, marls, and pitch

coals are characterized by increased resistance values

(to 200–300 

Ω m). The rocks of the Tueryk and

Koshagach suites are typically resistance differenti

ated. The resistivity of the Paleozoic and Vendian sed

imentary rocks ranges from 100 to 500 

Ω m (except for

limestones whose resistivity attains 1000 

Ω m and

higher). Magmatic rocks are characterized by the



resistivity from 500 to 5000 

Ω m.


Based on the analysis of a priori data, the main

interpretation model was determined as a four layer

cross section with a high resistivity upper part, a third

well conductive layer, and a nonconductive bottom

layer. The apparent resistivity curves corresponding to

this profile refer to the QH type (

ρ

1

 > 



ρ

2

 > 



ρ

3

 < 



ρ

4

).



The interpretation of the major part of the NF

TEMS data was carried out in the class of laterally

stratified models. Primarily, this is associated with the

high locality of the setup used for measurements

[Rabinovich, 1987; Metodicheskie, …, 1983]. Note

that the induction setups with coaxial loops are least

sensitive to nonhorizontal boundaries. The slopes of

the transmitting and receiving loops caused by the

topography of the Earth’s surface have a negligible

effect on the measurement errors.

The influence of the nonlateral boundaries and the

scarps in the basement manifests itself as an evident

distortion of individual segments in some sounding

curves (usually, the right hand branches of the appar

ent resistivity curves are distorted). These distortions

had been thoroughly analyzed earlier [Kuznetsov,

1982; Nevedrova et al., 2006]. If there are no grounds

to  take  into  account  the  phenomenon  of  induced

polarization, it is reasonable to determine the geoelec

tric parameters of the cross section from the curve

including the vicinity of the minimum and rejecting

the major part of the right hand side branch (other

wise we would not be able to reliably estimate the resis

tivity of the reference horizon).

In order to exemplify the data interpretation, we con

sider one of the NF TEMS field curves for profile 3 and

the corresponding geoelectrical model (Fig. 2). The

NF TEMS curve 102 completely corresponds to the

type described. The minimum of the curve indicates

the presence of a conducting layer in the section,

which overlies the reference geoelectrical horizon.

The inversion of the field data yielded a four layer

model with the resistivity of the third (lowest ohmic)

layer of 31 

Ω m; therefore, these sediments can be

referred to as the Koshagach suite. A shallower layer

with resistivity of 196 

Ω m characterizes the Tueryk

suite. The uppermost layer is characterized by the

highest resistivity attaining 1100 

Ω  m.  It  should  be

noted that the resistivity of the upper layer strongly

varies within the study area mainly depending on the

content of the coarse deposits, permafrost, and the

water content. The typical distortion of the right hand

branch  of  the  NF  TEMS  curve  102  is  apparent.  The

curves of apparent resistivity calculated for the strati

fied homogenous media with a nonconductive base

ment and plotted in bilogarithmic coordinates are

known  to  approach  a  line  inclined  at  an  angle  of

approximately 63

°

 to the horizontal axis with increas



62

IZVESTIYA, PHYSICS OF THE SOLID EARTH

  Vol. 47 

  No. 1   2011

 NEVEDROVA et al.

ing period. The distorted curves are usually character

ized by much higher angles. Therefore, a segment of

the right hand branch of the NF TEMS curve 102 was

disregarded in the determination of the geoelectric

parameters of the cross section, and the electric resis

tivity of the reference horizon was estimated condi

tionally.

Now, we turn to Fig. 3, which displays the field NF

TEMS curves for profile 4. This profile has a longer

extension than profile 3; it reflects the key features of

the tectonic depression. Here, several supposed fault

structures are distinguished. The geoelectrical NF

TEMS models 170 and 218 also contain four layers,

although the resistivity of these layers slightly differs

from the values for the profile3. NF TEMS station 218

is located in the bed of the Chagan–Uzun River. The

sediments in the upper part of the cross section are

most probably filled with water; their resistivity is less

than 200 

Ω m. The farther away from the river the

larger the resistivity of the layer. At station 170 it is

700

Ω m. Layers 2 and 3 are also composed of higher



conductive sediments than those in profile 3. This is

also determined by the geological conditions: most

probably, the sediments deposited closer to the center

of the depression are thinner layered.

The procedure for processing and interpretation of

the NF TEMS data acquired in the tectonic depres

sions has been already described in sufficient detail in

several works [Nevedrova, 2001; Nevedrova et al.,

2006]. Therefore, in the present paper we will focus on

the more detailed interpretation for the MTS method,

which has been first applied for the medium scale sur

vey in the Mountain Altai, and on the joint interpreta

tion of these two electromagnetic methods.

MTS Method

We start with the stage of qualitative interpretation,

when the dimensionality of the geological model is

selected. The real distribution of the MT field is

known to depend on all elements of the medium being

sounded, both vertical and lateral. Therefore, an

important stage of interpretation is the analysis of

magnetotelluric data, which allows us to construct the

interpretation model of the region under study. Here,

the leading role belongs to the polar diagrams of mag

netotelluric tensor 

 which represent the depen

dence of the MT responses on their orientation [Ber

dichevsky and Logunovich, 2005], and the magneto

telluric parameters, namely, the heterogeneity

parameter 



N [Berdichevsky et al., 1997], the skew

[Swift, 1967], and phase sensitive skew

η [Bahr, 1988].

The analysis of the polar diagrams of the imped

ance tensor for the western part of the Chuya Depres

sion showed that, generally, the cross section here is

quasi two dimensional (Fig. 4). In the two dimen

sional model striking along the 



Х axis

where the longitudinal and the transverse impedances



Z

 ||


 and Z

 are the principal values of the impedance



tensor. The apparent resistivity curves calculated along

the principal values of the impedance tensor are longi



Z

ˆ ,


Z

ˆ

0



Z

||

Z



0



,

=

100



0.1

1000


Ω m,

TEMS 102 (profile 3)

Geoelectrical

model


Rho,

1

2



3

4

Ω m,



1100

196


31

2000


H, m,

342


124

330


Theoretical

Observed


2

πt,  s



Fig. 2. Field data, synthetic curve, and geoelectrical model

at NF TEMS 102 (profile 3).

100

0.01


Ω, m

 tem 170 (profile 4)

Geoelectrical

model


Rho,

1

2



3

4

Ω m,



700

33

13



2000

H, m,

470


220

100


objective function = 1.47

0.001


Theoretical

Observed


t, s

100


0.01

Ω, m


tem 218 (profile 4)

Geoelectrical

model

Rho,


1

2

3



4

Ω m,


180

23

14



2000

H, m,

160


150

75

objective function = 4.74



0.001

t, s

Fig. 3. Field data, synthetic curves, and geoelectrical mod

els at NF TEMS 170 and 218 (profile 4).



IZVESTIYA, PHYSICS OF THE SOLID EARTH

  Vol. 47 

  No. 1   2011

INTERPRETATION OF COMPLEX ELECTROMAGNETIC DATA

63

tudinal (



ρ

||

) and transverse (



ρ

) with respect to the



strike of the geological structures. The validity of the

choice of the quasi two dimensional model is sup

ported by the analysis of magnetotelluric parameters

N, skew, and 

η (Fig. 5), which are calculated as

skew = 

where * denotes a complex conjugation.



It is known that in the laterally homogeneous

model 


N = skew = 

η.

The deviation of N from 0 characterizes the lateral



heterogeneity of the medium. In a two dimension

model, 


N 

≠ 0, while skew = η = 0. In a three dimen

sional model, all the three parameters are nonzero. As

follows from Fig. 5, at high frequencies (Т 

Ӷ 1), the

heterogeneity parameter N is less than 0.2, which indi

cates that one dimensional estimations are applicable

to assess the resistivity of the uppermost portion of the

geoelectric cross section. Starting with the periods

Т > 1 s, the value of N increases to 0.4–0.5, and with

decreasing frequency (



Т > 100 s), it increases up to 0.7.

High values of N correspond to an enhanced skew that

vary from 0.05 to 0.1 at long periods and attain 0.6 at

short periods. With increasing frequency, the phase

sensitive skew 

η increases, which, according to Bahr

[Bahr, 1988], is evidence of the absence of local three

dimensional inhomogeneities in the upper part of the

section. There are two zones where 

η decreases to 0.08

over the periods from 1 to 160 s. One is located in the

region of MTS stations nos. 18–15 and corresponds to

the zone of the deep fault; another (MTS sites nos. 6–

14) apparently reflects the fault structures of the

depression itself and its boundary with the Chagan–

Uzun block (Fig. 5).



N

1

4



Z

xx

Z

yy

Z

xy

Z

yx



Z



xy

Z

yx

(



)

2



,

=

Z



xx

Z

xy

+

Z



xy

Z

yx

,



η

0.5 Im Z



xy

Z

yy

*

Z



xx

Z

yx

*

+



(

)

Z



xy

Z

yx

,



=

Thus, the geoelectrical section of the study region

can be regarded as a regional two dimension structure

containing local three dimensional inclusions in the

middle and upper crust. The comparison between the

MTS and NF TEMS data described in the next sec

tion of the paper also confirms the two dimensionality

of the studied geoelectric section.

There are various recommendations and conclu

sions concerning which of the MTS curves are the

most informative in the 1D inversion [Kovtun, 2004;

Spichak, 1999; 



Sovremennye…, 2009]. The results dis

cussed in the present paper were obtained using the

longitudinal curves. They were selected because in the

study region the longitudinal MTS curves agree with

the NF TEMS curves, after having been overlapped by

the curves with which these MTS curves were inter

preted using the one dimensional programs.

The selection of longitudinal (quasi longitudinal)

curves over the study region was implemented in the

Line Inter MT program package, using which the

profile processing and interpretation of the MTS data

were carried out. Figure 6 shows the typical longitudi

nal curves calculated for the western part of the Chuya

Depression.

The main task of the further profile processing of

the results yielded by one dimensional inversion is

correction for the 

S effect and preparation of the data

for constructing the final geoelectrical sections.

As is well known, in the general case, if the

medium contains inhomogeneous inclusions at all

depth levels of the geoelectric cross section, the influ

ence of the 



S effect becomes stronger with the increas

ing depth of the MT field’s penetration into the Earth,

since ever new geoelectrical heterogeneities start

affecting the volume captured by the field.

This situation is typical in the region of study; i.e.,

the discrepancy in the 

ρ

т

 curves increases with the



increasing period of the electromagnetic wave.

Under these conditions, different corrections are

needed to compensate for the action of the 

S effect in

different depth intervals of the geoelectrical cross sec

tion. This procedure of introducing such corrections

has been implemented in the Line Inter TM program

complex. The key point in this procedure is calcula

MTS No. 1

MTS No. 2

MTS No. 3

MTS No. 4

MTS No. 5

MTS No. 6

MTS No. 7

MTS No. 8

MTS No. 9

MTS No. 10

MTS No. 11

MTS No.12

MTS No. 13

MTS No. 14

MTS No. 23

MTS No. 22

MТЗ No. 19

MTS No. 21

MTS No. 20

MTS No. 18

MTS No. 17

MTS No. 16

MTS No. 15

Profile line

Profile line

N

N

1



2



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